In this prospective observational trial, participants with chronic hemolysis will be assessed with echocardiogram for elevated tricuspid jet velocity and other evidence of pulmonary hypertension. Participants will have laboratory studies evaluating: severity of hemolysis, splenic function, inflammation, endothelial dysfunction, and hypercoagulability. There will be 3 main categories of participants enrolled in this study: (1) pediatric participants with severe sickle cell disease (SCD) (HbSS, HbS/β° thalassemia ) who are not receiving treatment (e.g., hydroxyurea or chronic transfusions); (2) pediatric participants with other forms of SCD or severe SCD (HbSS, HbS/β° thalassemia) patients being treated with hydroxyurea or chronic transfusions; and (3) pediatric and adult participants with other non-sickling hematological disorders.